1. Field of the Invention
The present invention relates to a cleaning method of a crucible which is used to contain a raw material melt in pulling a semiconductor single crystal by a Czochralski method. More particularly the present invention relates to a method for drying a crucible which is conducted in the last step of a cleaning process before the use of the crucible.
2. Related Art
Heretofore, in the case where a semiconductor single crystal is pulled by a Czochralski method, such a process has been adopted that raw material of the crystal is charged into a crucible, the crucible with the raw material is placed in a pulling furnace, the raw material is heated up to a temperature equal to or higher than the melting point to form a melt, thereafter a seed crystal is dipped into the melt and then the seed crystal is pulled upwardly from it. Crucibles used in the Czochralski method are made of high-purity quartz glass or the like, and they are shipped to customers from crucible makers after cleaning and drying in respective final processes. The crucibles each are delivered in a plastic bag in a sealed manner in order to avoid contamination during transportation.
When a crucible delivered is actually used at a customer, it is again cleaned with pure water (deionized water which has usually a resistivity of 10M .OMEGA. cm or higher) before the use in pulling a crystal for the purpose to remove contamination produced by friction with a plastic bag or contamination caused by poor sealing during transportation.
Cleaning before the use is very important for a high-purity quartz glass crucible used in pulling a silicon single crystal. The reason why is that a high-purity quartz glass crucible has a shiny, smooth inner surface, with which raw material contacts in the use, but the outer surface is a rough surface and quartz powder is with ease detached from the outer surface through friction between the crucible and the plastic bag during transportation though quartz raw material powder has been firmly adhered on the crucible. The detached quartz powder has a chance to stray onto the inner surface of the crucible and to adhere there. When a single crystal is pulled from a melt which is formed in such a contaminated crucible by heating silicon raw material charged in the crucible, a growing crystal comes to have dislocations caused by unmelted such detached quartz powder and to be disturbed in crystal growth. Accordingly, cleaning of a crucible before the use has been especially important for a crucible in pulling a silicon single crystal.
In such circumstances, a crucible has been again cleaned with pure water before the use at a site of the use after transportation, and then dried in order to remove adhered water on the surface by a dryer (for example, a dust free clean oven). A drying process with such a dryer is carried out in the following way: A crucible is placed in the dryer, a high-temperature air flow is circulated in the dryer, while wet air flow is partly exhausted to the outside through an exhaust duct. As alternatives, there are methods such as to blow a warm air flow on a crucible and to dry it by a infrared (IR) lamp.
When a crystal is pulled with a crucible which has not been dried enough, it takes a long time for gas replacement with an inert gas through vacuum in the pulling furnace, which reduces a working efficiency. Foreign matter is attached to a surface holding water with ease. Accordingly, it is necessary to dry the crucible enough.
In the case where crucibles are dried with a dryer after re-cleaning the crucibles before the use in a conventional manner, however, such drying and related processes need large-sized facilities. Such facilities cause a big loss in energy and require precaution on heat resistance of parts of their own, and, therefore, as a whole, cannot have been said to have an economically good efficiency. Even when warm air flow is adopted for the drying, there is a requirement for installation of a fan and the like, so that such drying needs a large-scaled facilities, too, and still loses a lot of energy. Furthermore, heating and drying by an infrared lamp cannot have been said to be an efficient, good drying method either, because of requirements for consideration on heat resistance of facilities and rotation of crucibles to achieve uniform heat distribution of a heating environment.